"The child is under the tree"
Translation:Mtoto yupo chini ya mti
These are locative verbs.
The first part is the same as the verbal prefix except that a- (he/she) changes to yu-. So u- is you (sg), yu is s/he, tu is we.
The second part can be -po, -ko or -mo. Mo deals with internal location, basically "to be in". Po and ko are a but harder to distinguish but "po" is generally for more specificity, at an exact point, and "ko" generally for more general location, "somewhere around" ... that's not the best explanation but it's a rough guide and it'll help you out in a lot of situations.